As an abrasive for finely polishing an optical glass and a crystal radiator in a finishing process, highly hard particles, such as diamond, boron nitride, silicon carbide, alumina, alumina-zirconia, zirconium oxide and cerium oxide, have been used.
Since these abrasives are hard particles, they are used in large quantities as an optical abrasive for a mirror finish of electronic parts, such as an optical lens, a semiconductor silicon substrate, a crystal wafer and a glass plate of a liquid crystal screen.
CMP (Chemical Mechanical Polishing) is applied as a method for polishing. In CMP, polishing is carried out in a state where an abrasive slurry lies between a polished object and an abrasive member such as an abrasive pad.
When a polished object contained in an abrasive slurry is increased through polishing, the finishing efficiency falls. Therefore, such an abrasive slurry will be disposed.
Some of the main components of an abrasive is obtained from minerals that are not produced in Japan, and thus partially relies on imported materials. In addition, many of the main components of an abrasive are expensive, and an abrasive is used in large quantities. Thus an abrasives is important and is one of resources that are eagerly desired to be reused.
In collection of an abrasive, it is important to remove components derived from a polished object from a collected slurry. For example, Patent Documents 1 and 2 disclose a method in which a dispersing agent or an electrolyte is added to dissolve components derived from a polished object.
However, these methods have a problem that foreign substances are likely to get mixed in a collected slurry, which decreases purity.